Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 3;~5350
MULTIFOCAL CONTACT LENS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a multifocal contact
lens.
Multifocal contact lenses, which are especially
intended for long-sighted persons, are designed so that
they simultaneously form several images of one and the
same object located in the field of vision. These various
images correspond to different dioptric corrections,
providing for good eyesight, both long-distance and
short-distance eyesight, with the selection between the
different images formed on the retina being made
spontaneously by the cortex.
(The term "long-distance eyesight" refers herein to
eyesight entailing accomodation to infinity or near
infinity. The term "short-distance eyesight" refers
herein to eyesight entailing maximum accomodation to the
subject or a neighbouring accomodation (typically for
reading) and the term "mid-distance eyesight" refers to
eyesight corresponding to any accomodation located
between these two extremes).
2. DescriPtion of the Prior Art
There are known lenses of this type (see, in
particular, EP-A-O 232 191) comprising a spherical
ring-shaped zone which provides long-sight correction and
an aspherical central zone which provides short-sight and
mid-sight correction, this central zone having a
dome-shaped surface of revolution bulging out on the
outer surface of the lens.
Since it has two zones, a spherical zone and an
aspherical zone, a lens of this type exhibits, on the
whole, dioptric power which varies from the center
towards ~he periphery. However, it is noted in practice
that while lenses of this type give good short-distance
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eyesight (through the central part of the aspherical
central zone) and yood long-distance eyesigh~ (through the
spherical ring-shaped zone), they are not very
satisfactory for mid-distance eyesight. This is because
of the transition in dioptric power between short-sight
correction and long-sight correction.
Apart from the fact that it does not give the wearer
satisfactory mid-distance eyesight, this drawback
increases his eye fatigue because of the difficulty
experienced by the cortex in selecting a satisfactory
image of an object located at mid-distance.
SUMMARY OF THE INVENTION
The present invention is aimed at coping with this
difficulty by proposing a multifocal contact lens
providing a very gradual transition between long-distance
eyesight and short-distance eyesight and, therefore,
providing excellent accommodation for mid-distance
eyesight.
As embodied and broadly described herein the
invention provides a multifocal contact lens comprising a
spherical ring-shaped zone to correct long sight and an
aspherical central zone to correct short sight and mid-
distance sight, said central zone having a surface of - ~-
revolution which is shaped like a dome bulging out of the -
external side of the lens, wherein the dome is
approximately cone-shaped, with its surface extending -
essentially on the external side of a cone. -
A preferred embodiment of the invention will be
described below with reference to the annexed drawings in
which: ~-
- FIG. 1 shows a partial view of the general shape of ;-
the external surface of the multifocal contact lens -
according to the present invention;
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- ~IG. 2 shows an enlargecl view, correspondin~ to
figure 1, ~7herein the scale of the ordinates has been -
considerably expanded so as to show the shape of the
central dome of the lens.
Very advantageously, the height of khe vertex of the
dome, before grinding and in the dry sta~e, ranges from 3
to 6 ym (from 3.57 to 7.14 ~m in the hydrated state) and
the diameter of the base of the dome, before grinding and
in the dry state, ranges from 1.4 to 1.9 mm (from 1.66 to
2.26 mm in the hydrated state).
Preferably, with reference to the lefthand part G of
FIG. 2, the generatrix of said surface of revolution
comprises, from its base to the vertex, a first arc of a -
curve with an increasing slope; a second arc of a curve -
with a decreasing slope, a third arc of a curve with a
substantially constant slope and a fourth arc of a curve
with a substantially increasing slope. : -
DESCRIPTION OF A PREFERRED EMBODIMENT
The lens comprises essentially (FIG. 1) a spherical
ring-shaped zone 10, giving long-sight correction, and a
aspherical central zone 20, giving short-sight correction
and mid-sight correction. ~ ~ -
The radius p of the spherical external surface 11 :~
(FIG. 2) of the ring-shaped zone 10 is chosen as a
function of the correction which is sought to be made for
long-sight correction. This radius typically having a
value ranging from 6.60 to 9.30 mm. for corrections ~
respectively ranging from +5 and -11 diopters. ~ :
The central zone 20 has a surface 21 that bulges out .:
from the spherical surface 11, this surface 21 being a
dome-shaped aspherical surface of revolution. -
We shall now give a detailed description of the shape
of this aspherical surface 21 with reference to figure 2
where, for the clarity of the illustration, the scale of -
the heights ~ has been considerably increased, i.e. by a ~
coefficient of about 100 times. ~ -
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Furthermore, all the dimensions shown will always be
dimensions obtained after grinding, hence they will be
S dimensions in the dry state. To obtain the dimensions in
the hydrated state, the linear expansion coefficient
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corresponding to the material used, in the case of
flexible lenses, must be applied to the values indicated.
For the optical functions of the lens it is, of course,
the dimensions of the flexible lens, namely the lens in
~; 5 the hydrated state, that are the determining dimensions.
To make the lens of the invention, it is possible,
for example, to use a poly(hydroxyethylmethacrylate) with
38.4~ hydrophyly, displaying an isotropic linear
expansion coefficient of 1.19.
Finally, it will be noted that the shape of the
curve that will be given is the one obtained before
grinding. This means that its general shape will be -
preserved after grinding but that certain transition
zones become even more gradual, especially the vertex of
the dome as well as its junction with the spherical
ring-shaped zone.
The dome-shaped surface of revolution 21, which
forms the outer surface of the aspherical central part of
the lens~ is substantially shaped like a cone 22, but has
its surface extending essentially on one and the same
side of this cone, essentially to the exterior of this
cone.
In other words, the generatrix of the surface 21,
(namely the arc AE) will be essentially located on the
same side of the generatrix of the cone 22 (namely the
- segment AE) having the same vertex and the same base ;~
~- circle.
The two main parameters that determine the shape of
the dome are, before grinding and in the dry state: -
- the height OE of the vertex which should range
between 3 and 6 um (5.75 ~m in the example of FIG. 2),
and
- the diameter of the base circle ~that is twice the
~ radius OA) which shculd range between 1.4 and 1.9 mm
; 35 (1.58 mm in the example of FIG. 2)
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As for the contour of the generatrix of the dome, it
is sub-divided essentially into four distinct parts which
can be analyzed as follows, with reference to the
lefthand side G of FIG. 2
A first arc of a curve AB actually generates a
I junction zone between the spherical ring-shaped zone of
the lens and the aspherical central portion which has a
true mid-sight and short-sight correcting function.
A second arc of a curve BC generates a transition
zone which initiates the mid-sight correcting zone.
A third arc of a curve CD, which is in fact
essentially rectilinear, generates the mid-sight
correcting zone, possibly, the short-sight correcting
zone.
lS Finally, a fourth arc of a curve DE generates the
central portion of the dome which is responsible for
eyesight correction for short distances to very short
distances.
It must be specified here, with reference to the
left-hand side G of FIG. 2, that the contour of the
generatrix of the dome has a derivate dH/dR with a
constantly positive sign, the different single arcs of
curves being capable, however, of having increasing,
constant or decreasing slopes.
The first arc of a curve AB has an increasing part;
the generated zone does not have any natural optical
corrective function, but is, however, indispensable to
preventing the perception of spurious images.
The coordinates of the point B can be defined by its
height HB and its radial spread Rg, with
reference to the main characteristics of the dome, i.e.,
respectively its height OE and its base radius OA, in the
following way:
0.7 ~ Rg/OA ~ 0.8
0.2 ~ Hg/OD ~ 0.3
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The second arc of a curve BC has a decreasing slope.
However, in the specific embodiment described with
reference to figure 2, it touches a maximum, i.e.
substantially in the middle of the arc BC, the spread
with respect to the generatrix of the cone (segment AE)
touches a maximum. This zone is essential to initiate or ;~
prepare the subsequent mid-sight correction zone.
The coordinates of the point C may be defined by its
height Hc and its radial spread RC with reference
to the main characteristics of the dome, namely its
height OE and its base radius OA, as follows:
0.55 ~ RC/OA ~0.65
0.40 ~ HC/OA ~0.50
The third curve position CD has a substantially
constant slope, i.e. it actually generates a surface
which is practically truncated and is responsible for
mid-sight correction.
The coordinates of the point D can be defined by its
height HD and its radial spread RD with reference
to the main characteristics of the dome, namely, its
height OE and its base radius OA, respectively, as
follows:
0.15 ~RD/OA ~ 0.25
0.90 ~HD/OE ~ 0.95
; 25 Finally, the last arc of a curve DE, with a
substantially increasing slope, creates a substantially
- conical last surface portion. The exact position of the
point D as well as the slope of this arc of a curve may
undergo a few variations, essentially depending on the
short-sight corrective power desired for the lens. In
practice, especially after subsequent grinding, the
vertex of the dome may have a slightly rounded appearance
without this resulting in any disturbance of
short-distance eyesight.
; 35 Lenses of this type can be made by machining a disk
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of material conventionally used to produce contact lenses
or, again, by molding. In either case, semi-finished
, lenses are obtained and generally have to undergo final
grinding.
Experiments on contact lenses according to the
present invention have shown that, unlike all multifocal
lenses proposed until now, they can be used to give
I perfect long-distance eyesight, short-distance eyesight
as well as mid-distance eyesight without the least
perception of spurious images.
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